In 2005, Van trappen et al., isolated the bacterium from sediments in Shem Creek, Germany, and a salt marsh in Charleston Harbor, USA.[13] Since its discovery in 1919, the genus Alcaligenes has undergone many changes. Besides A. aquatilis, does the genus also contain for example Alcaligenes faecalis. In the years since its discovery, several strains of A. aquatilis have been isolated from a variety of locations. Strain DQ168 was isolated from oil-polluted sediments in Quintero Bay, Chile, while strain 393 was isolated from a sink in a Pakistaniintensive care unit.[14]
Genome
A. aquatilis has a circular genome with an average size of 4.14 megabase base pairs (Mbp) and a GC content of 56.6%,[10] with variance between strains. For example, strain DQ168 has a genome size of 4,323,879 bp.[10] A genome annotation for DQ168 found 3,892 coding sequences.[5] Complete genome sequences are available on the GenBank, European Nucleotide Archive, and DNA Data Bank of Japan websites.[5]
Morphology/biochemistry
A. aquatilis is a rod-shaped, Gram-negative, motile bacteria with a cell length of 1.75 micrometer (μm) and a cell width of 0.9 μm.[3] It has been shown to grow in temperatures ranging from 4-37°C, with optimal growth at 21°C.[3]A. aquatilis forms circular colonies that are either yellow or non-pigmented, depending on the strain.[3]
Applications
Nitrogen elimination
Heterotrophic nitrification-aerobic denitrification (HNAD) is a process where nitrogen and organic carbon are removed under aerobic conditions.[15] HNAD can improve nitrogen removal in biological wastewater treatment by functioning under completely aerobic conditions, balancing pH through simultaneous denitrification, saving space and construction costs, and reducing both energy consumption and carbon emissions. The efficiency of an HNAD performing bacterium depends on the range of carbon sources it can utilize as well as the pH it can survive in.[15]A. aquatilis isolated from aerobic activated sludge was found to have HNAD capabilities.[6]
A. aquatilis was found to proliferate in piggery wastewater, its abundance increasing with wastewater treatment,[6] suggesting that A. aquatilis has the potential to significantly increase the ammonia and total nitrogen removal in piggery wastewater treatment, even at high ammonia concentrations.[6]A. aquatilis has rapid growth rates and can utilize various carbon substrates for electrons and energy, making the microbe advantageous for Biological Nitrogen Removal (BNR) systems. In piggery wastewater with ammonium nitrogen (NH4+-N), ammonia removal efficiency was as high as 90% after 24 hours of cultivation with A. aquatilis, and then increased further to 95.5% after 36 hours.[6] In comparison, the control condition without A. aquatilis had the highest NH4+-N removal efficiency of only 59.2% after 72 hours of treatment.[6]
The interaction between A. aquatilis and cyanobacteria has garnered attention due to its potential implication in algal biotechnology.[7] During co-cultivation, studies have shown A. aquatilis to exhibit antagonistic effects on cyanobacteria by producing compounds such as tannins, amino acids, and hydroxamate siderophores. Along with enzyme activities of cellulase and protease, this interaction has been found to inhibit the growth of cyanobacteria and cause a decrease in its biomass.[7]
Zone inhibition assays using BG-11 medium, an increase in the concentration of A. aquatilis resulted in repressed or decreased growth of cyanobacteria. According to a relevant T-test, an A. aquatilis density of 8.9x10^6 cells mL^-1 resulted in a 49.83% decrease in cyanobacteria biomass after an incubation period.[7]
Ultimately, the mechanism of inhibiting cyanobacteria appears to involve algicide activity by bacteria associated with A. aquatilis. In a study, when inoculated with cyanobacteria, A. aquatilis formed a layer of cells around called the inhibition zone.[7] This suggests that A. aquatilis does not directly attach to the cyanobacteria to inhibit its function, but rather releases substances or relies on other associated bacteria to inactivate cyanobacteria activity.
The potential control of cyanobacteria with the use of A. aquatilis is significant, as cyanobacteria are capable of secreting toxic substances that would negatively impact marine systems. However, its use in environmental or industrial settings are still limited due to factors such as the specificity of its antagonistic activity and the need for further research to study long-term efficacy and safety.
Desalination
To address issues of freshwater scarcity in some areas, there have been significant advancements in the development of desalination technology to remove ions from seawater.[12] One primary concern is scale deposits (magnesium ion [Mg2+] and calcium ion [Ca2+]) formed during desalination, particularly affecting equipment used in reverse-osmosis processes.[18]A. aquatilis may be a viable pre-treatment step in desalination, as it removes Ca2+ and Mg2+ from seawater through microbial-induced carbonate precipitation (MICP).[12] The surface of A. aquatilis contains a negatively charged extracellular polymeric substance (EPS) that adsorbs to the positively-charged Ca2+ and Mg2+, removing the cations with efficiencies as high as 93% and 72%, respectively.[12]
Heavy metals in seawater are also of concern due to their toxicity and the difficulty of their removal. Through MICP, A.aquatilis has the potential to break down proteins and amino acids in the water, which releases ammonium and carbonate.[12] The EPSs can facilitate nickel-carbonate precipitation, especially under basic conditions, removing the dissolved nickel (Ni) by as much as 92%.[12]
One concern with MICP as a removal method is that it generates ammonia as a potentially toxic byproduct. However, A. aquatilis can convert ammonia to nitrogen gas under aerobic conditions.[12] This unique feature may reduce the need for additional desalination and pollution-removal processes.
Decolorization ability
Synthetic organic dyes are widely used in textiles, tanning, cosmetic products, and paper industries. Although they are considered micropollutants, many synthetic organic dyes are toxic to marine organisms, obstruct light penetration in aquatic ecosystems, and reduce oxygen transfer.[19] This presents a risk to both aquatic ecosystems and higher-order consumers such as humans.[20]
Azo dyes are the largest class of synthetic dyes. A. aquatilis can successfully decolorize 82% of Synazol red 6HBN, an azo dye, after a 4-day static incubation at 37°C and pH 7.[8] The mechanism is hypothesized to be a reduction of the nitrogen triple bond and cleavage to produce various end products. Desulfonation reaction, oxidative deamination, and carboxylation produce intermediate products which can be used by the bacterium as substrates in amino acid metabolism and Krebs Cycle intermediates. Decolorization promotes an increased growth rate of A. aquatilis. The decolorized wastewater does not impede the growth of other microbes.
Furthermore, one study found that the A. aquatilis type strain LMG 22996 reduced color by 86% when cultured in wastewater, which was abundant in synthetic dyes from textiles.[22] The culture was inoculated with cow dung, jaggery, and urea to assess the bioremediation efficiency. A. aquatilis successfully reduced the biological and chemical oxygen demands by 70% and 81%, respectively. This suggests that this bacterium may be a viable treatment option for polluted environments such as oxygen dead zones.
The unique ability of A. aquatilis to decolorize synthetic dyes by breaking down complex structures through its metabolic pathways, while simultaneously reducing pollution parameters and supporting biotic growth, supports the bacterium as a potential tool for bioremediation. This makes it a potential treatment for dye-contaminated effluents in textile industry hubs such as China, Bangladesh, Vietnam, and India.[23]A. aquatilis could potentially reduce the impact of synthetic dye pollution on aquatic ecosystems.
Hydrocarbon breakdown
Genetic analysis of A. aquatilis has indicated the potential for the breakdown of hydrocarbons, an important environmental pollutant. The QD168 strain contains the genes for 7 aromaticcatabolic pathways and 16 peripheral pathways,[10] while analysis of the BU33N strain found 97 genes related to the breakdown of aromatic compounds.[11]
A beach in Prince William Sound is polluted by the Exxon Valdez oil spill.
Growth assays of A. aquatilis show the bacterium's ability to metabolize numerous compounds found in pollutants such as crude oil, as well as other benzene derivatives and straight-chain hydrocarbons.[10][11] Strain BU33N displayed clear emulsification activity, indicating the production of biosurfactants, which reduce tensions in hydrocarbons, allowing for their breakdown.[11]
The breakdown of benzene is of particular interest as it is present in oil-polluted sites and is extremely toxic.[5] Through transcriptional analysis, it was found that A. aquatilis likely catalyzes benzene by converting it to a phenol intermediate, and then into catechol.[10] From catechol, A. aquatilis uses six enzymes to form acetyl-CoA and succinyl-CoA.[11]
The ability of A. aquatilis to break down pollutants associated with oil spills suggests the potential for use in cleaning up oil spills in aquatic environments and mitigating industrial contamination in coastal regions.[10][11]
Degradation of pharmaceutical drugs
A. aquatilis has been shown to use Diclofenac (DCF) as a carbon source.[9] DCF is a common pharmaceutical non-steroidal anti-inflammatory drug (NSAID) used to treat pain, fever, and inflammation. It enters wastewater after human metabolism and excretion, and is highly present in aquatic environments.[24] DCF induces cellular toxicity by increasing the production of reactive oxygen species, making this metabolic feature particularly unusual. However, the exact mechanism behind this remains unclear and future research is required to solidify the practical application of A. aquatilis in bioremediation of pharmaceutical pollution.
